The present invention is broadly directed to ion-selective electrodes (ISE""s). More specifically, it relates to polymeric compositions for internal reference electrodes of dry-operative ISE""s.
A wide variety of ion-selective electrodes are known for measuring the amount of an ion in solution. Typically devices for obtaining such measurements include a reference or standard electrode and a separate ion-selective electrode. When simultaneously contacting a solution to be analyzed, the reference or standard electrode and the ion-selective electrode together constitute an electrochemical cell across which a potential difference develops in proportion to the logarithm of the activity of the ion to which the ion-selective electrode is sensitive. The activity is related to the concentration of the ion in the solution. The relationship between potential difference and ion activity in solution is described by the well-known Nernst equation. An electrometric device, usually either a direct reading circuit or a null-balance potentiometric circuit, is employed for measuring the potential between the electrodes.
In principle, an ion-selective electrode can be constructed which is sensitive to any cationic or anionic substance. Cations that can be determined include, but are not limited to, group IA ions, such as sodium, potassium, lithium (alkali metals) and hydrogen; group IIA metal ions such as calcium and magnesium (alkaline earths); metal ions from groups VIA, VIIA, VIIIA, IB, IIB and IIIB; and lead ion from group IVB. Anions include, but are not limited to, halide ions, chloride and fluoride being of particular interest. Carbon dioxide, which is non-ionic, can be determined using an ISE sensitive to hydrogen ion.
Ionic substances are present in a wide range of sample types, including but not limited to, industrial effluents, tap water, rain water, sewer water, biological sources, such as plant and animal derived fluids, and so forth. Human biological fluids are of particular interest and include whole blood, serum, plasma, saliva, sweat, bronchial fluid, vaginal excretions, and so on.
Electrodes based on ion-selective glass membranes are well-known. Solid-state electrodes are also known, such as those described in U.S. Pat. No. 3,856,649 to
Genshaw et al. (the ""649 patent) and in a paper entitled xe2x80x9cMiniature Solid State Potassium Electrode for Serum Analysisxe2x80x9d in Analytical Chemistry, v45, pp 1782-84 (1973). An advance in solid-state electrodes has been achieved with the xe2x80x9cdry-operativexe2x80x9d electrodes described in U.S. Pat. No. 4,214,968 to Battaglia et. al. (the ""968 patent), U.S. Pat. No. 4,053,381 to Hamblen et al. and U.S. Pat. No. 4,487,679 to Stare.
Metal, insoluble metal salt solid-state electrodes comprise an electrically conductive inner element, a metal, having disposed thereon an insoluble salt of the metal. The metal, insoluble metal salt combination represents a half-cell or in the context of the present invention, an internal reference electrode, which can be used directly by contacting it with a solution containing an ion to be measured. Alternatively, the internal reference electrode can comprise in intimate contact with the metal and insoluble metal salt, a water-soluble salt dispersed in a hydrophilic xe2x80x9cbinderxe2x80x9d capable of forming a solid matrix. The anion of the water-soluble salt has the same identity as the anion of the insoluble metal salt. In intimate contact with the matrix of the internal reference electrode is a hydrophobic zone which shields the internal reference electrode from direct contact with the ion-containing solution. The hydrophobic zone generally comprises an ion-specific carrier.
The ""649 patent describes the use of polyvinyl alcohol as binder to form a hydrophilic layer which includes a water-soluble salt.
The ""968 patent lists several binders for the xe2x80x9cdriedxe2x80x9d reference electrolyte solution: polyvinyl alcohol, gelatin, agarose, deionized gelatin, polyacrylamide, polyvinyl pyrrolidone, poly(hydroxyethyl acrylate), poly(hydroxyethyl methacrylate) and poly(acrylic acid). Deionized gelatin is a preferred binder in the ""968 patent. Unfortunately, in dry-operative ISE""s, the layer comprising the internal reference electrode having gelatin as binder, is brittle, it does not adhere well to the hydrophobic overlayer, and is sensitive to variations in humidity. As a result, the integrity and performance of these ISE""s are adversely affected.
We have found that improved dry-operative ion-selective electrode performance can be achieved using as binder, copolymers prepared from a hydrophilic monomer having at least one carboxylic acid group or salt thereof and a hydrophobic monomer. The resulting copolymer has a glass transition temperature lower than the acid homopolymer, provides good interlayer adhesion as well as high salt tolerance.
In one aspect the present invention relates to dry-operative ion-selective electrodes comprising:
a) an internal reference electrode comprising a water-soluble salt dispersed in a polymer consisting essentially of 60 to 99 weight percent of a monomer having at least one carboxyl group or salt thereof and 1 to 40 weight percent of a hydrophobic monomer; and
b) a hydrophobic zone in contact with the internal reference electrode, said hydrophobic zone having distributed therein a carrier selective for the ion.
The internal reference electrode can be a metal salt, insoluble metal salt type electrode, or an oxidation-reduction type electrode comprising a metal and a redox salt couple. The dry-operative ion-selective electrode may further comprise a support wherein the internal reference electrode is disposed between the support and the hydrophobic zone. In preferred embodiments, the binder polymer comprises 70 to 95 weight percent of a monomer having at least one carboxyl group or salt thereof and 5 to 30 weight percent of a hydrophobic monomer.
Preferred polymers of the invention consist essentially of a monomer of formula 
wherein R1 is H or xe2x80x94COOM; R2 is H, Cl or xe2x80x94COOM; R3 is xe2x80x94COOM, xe2x80x94CH2CH2COOM, xe2x80x94CHCONHC(CH3)2CH2COOM, or 
wherein R4, R5, R6, R7 are independently H, methyl, or ethyl, and a monomer of formula 
wherein R8 is H or methyl; R9 is methoxy, ethoxy, propoxy, butoxy, hexoxy or xe2x80x94NHCH3.
In another aspect the present invention relates to a method for determining the presence or amount of an ion in a liquid comprising:
A) contacting a dry-operative ion-selective first electrode with a sample of the liquid wherein the dry-operative first electrode comprises
a) an internal reference electrode comprising a water-soluble salt dispersed in a polymer consisting essentially of 60 to 99 weight percent of a monomer having at least one carboxyl group or salt thereof and 1 to 40 weight percent of a hydrophobic monomer; and
b) a hydrophobic zone in contact with the internal reference electrode, said hydrophobic zone having distributed therein a carrier selective for the ion,
B) contacting a second electrode with a solution comprising a known or constant amount of an ion to which said second electrode is selective, and wherein said dry-operative first electrode and said second electrode are in electrochemical contact or are capable of being in electrochemical contact; or
C) contacting the dry-operative ion-selective first electrode and the second electrode with the same sample of the liquid wherein the sample comprises a known or constant amount of an ion to which said second electrode is selective; and
D) measuring the potential difference between the dry-operative first electrode and the second electrode as a determination of the presence or amount of the ion in the liquid sample.
The second electrode in the above method of determining an ion can be any suitable reference electrode, such as a calomel electrode or others known in the art, or a solid-state electrode, such as a dry-operative electrode. It can be identical in structure and composition to the dry-operative ion-selective first electrode. If the first and second electrodes are selective for the same type of ion then the solution of part B comprises a known or constant amount of said ion. If the second electrode is selective for a second ion type that is different from that to which the first electrode is selective, then the solution comprises the second ion type in a known or constant amount. If the method for determining the presence or amount of an ion is conducted as in part C, then the sample comprises a known or constant amount of a second ion to which the second electrode is selective and which is different from the ion to which the first electrode is selective.
As ISE technology is well-known and described in numerous publications, only a brief account of the construction and use of dry-operative electrodes will be provided. Specific details can be found, for instance, in U.S. Pat. Nos. 4,214,968, 4,053,381 and 4,487,679. The invention is illustrated by reference to a preferred embodiment, a metal, insoluble metal salt type electrode. However, the invention is not limited to this type of electrode.
The internal reference electrode can comprise any metal that is commonly used for this purpose; preferably one that readily forms an insoluble salt and has good electrical properties. Such metals include silver, copper, lead, amalgams and the like. The insoluble salt is disposed on a surface portion of the metal and has, as cation, the cation form of the metal. The anion of the insoluble salt is generally a halide or a sulfide. The insoluble salt may be formed by anodizing the metal in a suitable solution, by a physical application of a dispersion of the salt in a suitable carrier that will adhere to the metal or by other suitable methods. The salt may be formed about an end portion of the metal or its position may be varied according to the desired structural features of the electrode. The water-soluble salt is in intimate contact with the metal and insoluble metal salt. It generally has as cation, an alkali or alkaline earth metal, such as sodium, potassium, magnesium, calcium, and barium; and as anion, a halide. Representative salts include but are not limited to NaCl, KCl, KBr, MgCl2, and BaCl2. The water-soluble salt is dispersed in a solid-forming hydrophilic binder, as discussed above. In contact with the binder is a hydrophobic zone. The hydrophobic zone can be formed from any suitable hydrophobic polymeric material such as polyvinyl chloride (PVC),polyvinyl acetate, polymethylmethacrylate, polyvinylidene chloride, polystyrene and the like. An ion-selective carrier dispersed in the hydrophobic zone renders the electrode specific for an ion of choice. A large number of carriers selective for specific ions are known, including but not limited to: valinomycin, which is selective for potassium; cyclic polyethers of various constitution which make the electrode selective for lithium, rubidium, potassium, cesium or sodium ions; tetralactones; biscyclic ethers; cryptands; hemispherands; calixarenes; cyclic amides; macrolide actins(monactin, nonactin, dinactin, trinactin), the enniatin group (enniatin A,B), cyclohexadepsipeptides, gramicidine, nigericin, dianemycin, nystatin, monensin, esters of monensin (especially methyl monensin for sodium ion), antamanide, and alamethicin (cyclic polypeptides); magnesium or zinc uranyl acetate; 6,8-dichlorobenzoylene urea; didecylphosphoric acid-dioctyl phenylphosphonate; tetraphenylboron; tridodecylhexadecylammonium nitrate; and 4-amino-4xe2x80x2-chlorodiphenylhydrochloride barium salt. Other useful carriers are described by Amman et al. in Helv. Chim. Acta, v58, p535 (1975). Useful calcium ion selective electrodes can be prepared using antibiotic A-23187 as the ion carrier and tris(2-ethyl hexyl) phosphate, tri(m-tolyl)phosphate, or dioctyl phenyl phosphonate as the carrier solvent. (See Pressman, B. C., Annual Review of Biochemistry, E. B. Snell, ed., V5, 1976, pp. 501-503). Thus, electrodes can be prepared which are selective for potassium ion, sodium ion, lithium ion, magnesium ion, calcium ion, ammonium ion, hydrogen ion, cesium ion, bromide ion, chloride ion, fluoride ion, or iodide ion, carbonate ion, salicylate ion, nitrate ion, and so forth.
In preferred embodiments the ion-selective electrodes of the invention are multilayered and include a support which may be comprised of any suitable material, such as ceramic, wood, glass, metal, paper or cast, extruded or molded plastic or polymeric materials, and so on.
The presence or amount of an ion in a solution can be determined by measuring the difference in electrical potential (potential difference) between solution 1 and solution 2 (both usually aqueous) in a cell arrangement schematically represented by the following:
electrode 1/solution 1//solution 2/electrode 2
The activity or concentration of the ion of interest in solution 2 (in this case, the solution of unknown concentration) can be derived from the measured potential difference through use of the well-known Nernst equation. Alternatively, any algorithm or method relating the measured potential difference to the amount or effective amount of the ion can be used. Electrode 2 can be a dry-operative ion-selective electrode of the present invention. Electrode 1 can be any suitable reference electrode or standard electrode such as a saturated calomel electrode. It can be a dry-operative ISE of the present invention. Solution 1 can comprise a known amount of the ion whose activity or amount is unknown in solution 2.
The activity or amount of an ion can also can also be determined by measuring the potential difference between two electrodes contacting a single solution containing the ion using a so-called junctionless arrangement represented by the following:
electrode 1/solution/electrode 2
wherein electrode 1 is an ion-selective electrode specific for an ion in the solution. Electrode 2 can be a reference electrode or an ion-selective electrode specific for a different ionic species which is present at a known or constant level in the solution.